Heretofore, there have been many different types of motor protectors made in large quantities, and a number of different designs of these motor protectors have been provided. In use, these motor protectors must operate at predetermined temperatures for circuit opening and closing action, and they must remain operative and functioning in accordance with the desired calibration under widely variable ambient temperatures. Furthermore, in small electrical control instruments of this type, very high quality standards are required and very severe test conditions are set-up for these articles by safety testing labs, such as Underwriters Laboratory, Inc. The controls must open the controlled circuit, for example, in less than 45 seconds at 180.degree. F., 76.degree. F., and at -20.degree. F., and in more than 11/2 seconds. A constant current motor is used in such test. The current drawn however, by such motor is maximum when started. The current reduces gradually for about 40 seconds and then stabilizes.
In other prior constructions, bimetal strips have been wound with wire when the wire is a heater to accelerate bimetal action. This type of a heater is costly to assemble and it presents electrical insulation problems.